Rubber compositions containing diene-based elastomers often contain reinforcing fillers such as for example rubber reinforcing carbon black and precipitated silica together with a coupling agent for the precipitated silica. Rubber tires may contain at least one component comprised of such rubber composition.
Sometimes it may be desirable to provide a rubber composition containing an alternative reinforcing filler.
For example, such additional, or alternative, reinforcing filler may be in a form of graphene or carbon nanotubes.
Graphene and carbon nanotubes may exhibit exceptional mechanical and electrical properties that make them very interesting for the use in rubber compositions including for tire components. However, in order to benefit from the advantages of graphene or carbon nanotubes, it is important for a high level of their dispersion in their associated rubber be promoted. Such dispersion is generally a challenge because graphene sheets tend to stack together, exfoliated graphene platelets tend to agglomerate and carbon nanotubes tend to from entangled aggregates to thereby form restricted dispersions in the rubber composition and thereby weak interfacial interactions with diene-based elastomers in the rubber composition.
Therefore, it is recognized that more effective dispersions of graphene and carbon nanotubes in rubber compositions containing diene-based elastomer is desired.
For this invention, it is desired to promote interfacial bonding of graphene and carbon nanotubes to diene-based elastomers in a rubber composition.
For this endeavor, it is proposed to utilize coupling agents to promote coupling graphene and carbon nanotubes to diene-based elastomers with a coupling agent having a moiety to promote a pi-pi (π-π) network interaction with the graphene or carbon nanotubes and another moiety, such as for example a polysulfide, thiol (e.g. mercapto) moiety, or carbon-carbon double bonds, which is interactive with the diene-based elastomer(s) contained in a rubber composition.
However, the presence of a polysulfide or thiol (e.g. mercapto) moiety on a coupling agent is envisioned as rendering the coupling agent to be excessively and undesirably reactive with the diene-based elastomer(s) in the rubber composition during a preliminary rubber mixing stage under high shear and high temperature mixing of the rubber composition prior to addition of sulfur curatives.
Therefore, it is proposed to chemically block the polysulfide or thiol moiety of the coupling agents from prematurely interacting with diene-based elastomer(s) in the rubber composition.
For such purpose, it is proposed to chemically block the thiol moiety (e.g. mercapto moiety) of the coupling agent with a protective compound containing at least one of dithiobenzoate, trithiocarbonate, thioester, and polysulfide groups containing from about 2 to about 4 connecting sulfur atoms.
Representative of such compounds are, for example, 1-pyrene-methyl-trithiocarbonate, 1-phenanthrene methyl trithiocarbonate, and anthracene-9-methyl trithiocarbonate.
In order to promote coupling of the graphene or carbon nanotubes it will be necessary to unblock the blocked polysulfide or thiol moiety of the coupling agent during the rubber mixing process so that the coupling agent may interact with the diene-based elastomer(s) in the rubber composition.
It is therefore a significant aspect of this invention is to controllably promote unblocking of the coupling agent during mixing of the rubber composition which contains the graphene or carbon nanotubes.
Historically, graphene may be provided in a form of exfoliated graphite platelets, referred to herein as graphene, from exfoliated intercalated graphite (exfoliated intercalated graphite in a stacked platelet form with internal galleries between the graphite platelets) which may be exfoliated, for example, chemically or thermally. The graphene has been suggested, for example, for use in rubber compositions for various tire components. For example, and not intended to be limiting, see U.S. Pat. Nos. 7,479,516, 7,224,407 and 6,892,771 and U.S. Patent Publication No. 2006/0229404.
Such graphene (exfoliated graphite platelets) are typically irregularly shaped platelets and nano-sized in a sense that they have an average thickness in a range of from about 1 nm to about 5 nm (nanometers) and an average lateral dimension in a range of from about 0.1 to about 1 micrometer (e.g. in a range of from about 0.01 to about 1 square micrometers which is envisioned to have, for example, an average surface area per gram in a range of from about 20 to about 800 square meters per gram).
Historically, carbon nanotubes or graphene have heretofore been suggested for inclusion in rubber compositions, including tire treads, for various purposes. For example, and not intended to be limiting, see Patent Publications U.S. Pat. No. 6,476,154, US 2006/0061011, US 2010/0078194, US 2011/0146859, WO 2003/060002, DE 102007056689, JP 2009/046547, KR 100635604 and KR 2005027415.
Such carbon nanotubes are nano-sized particles in a sense of having, for example, an average diameter or thickness in a range of from about 1 nm to about 100 nm and an average L/D (length to diameter or thickness dimension, or ratio) in a range of from about 10/1 to about 10,000/1.
Such carbon nanotubes may be, for example, a product of gaseous carbon-containing compound such as for example, at least one of acetylene and ethanol, usually contained in nitrogen or hydrogen passed through or over a heated catalyst (e.g. heated to about 700° C.) of metal nanoparticles. Carbon deposited on the metallic nanoparticles in a form of the carbon nanotubes is recovered.
As indicated, a significant aspect of this invention is to provide the graphene or carbon nanotubes together with a coupling agent to controllably promote their interaction with diene-based elastomer in a rubber composition and thereby promote reinforcement of the rubber composition.
In the description of this invention, the term “phr” is used to designate parts by weight of a material per 100 parts by weight of elastomer. The terms “rubber” and “elastomer” may be used interchangeably unless otherwise indicated. The terms “vulcanized” and “cured” may be used interchangeably, as well as “unvulcanized” or “uncured”, unless otherwise indicated.